1. Field of the Invention
This invention relates to electrical insulating oils and is particularly concerned with oils for use in transformers, reactors, circuit breakers, switches, cables and other equipment which are characterized by better electrical properties than oils available heretofore.
2. Description of the Prior Art
Electrical insulating oils used in transformers, reactors, circuit breakers, switches, cables and other equipment for insulation and cooling purposes are generally manufactured from crude oil distillates or desaphalted petroleum fractions by treating the selected feedstocks for the removal of sulfur, oxygen, nitrogen and other constituents which may adversely affect their electrical properties and oxidation resistance. The processes used include acid treating, hydrogen refining, solvent extraction, dewaxing, and treatment with clay or other adsorbents. It has been shown that the properties of such oils can be improved by limiting their aromatic contents and that the use of oxidation inhibitors and other additives is also helpful in preventing the degradation of such oils in service.
Although the methods referred to above generally permit the preparation of insulating oils which have reasonably high oxidation resistance and relatively long service life under normal conditions, experience has shown that the electrical properties of such oils often leave much to be desired. An oil which is to be used as an insulating oil in a transformer, switching system or similar device must be capable of resisting current at much higher voltage levels than those at which the device is normally operated. Lightning and other disturbances may cause voltage surges sufficiently high to break down the oil and damage the equipment. The ability of an oil to resist such surges, referred to as its impulse strength, is measured in terms of both positive and negative waves. Similarly, the ability of an oil to withstand direct current voltages is measured in terms of both the direct current positive breakdown voltage and the direct current negative breakdown voltage. Conventional electrical insulating oils generally have moderately high negative impulse strength and direct current breakdown voltage values and somewhat lower positive impulse strength and direct current breakdown voltage values. Efforts to improve these normally result in an improvement in one at the expense of the other. Oils which have both high positive and high negative impulse strength and direct current breakdown voltage values have not generally been available in the past.
Another electrical property which is of importance in insulating oils is the gassing tendency of the oil. Many conventional oils will emit hydrogen gas when subjected to an intense electrical field sufficient to cause corona or partial discharging. If a gas bubble lodges in a critical location between conductors in the windings for example, the total resistance between the windings may be lowered to such an extent that an arc may occur and cause substantial damage to the equipment. If such an oil is used in a closed system, the liberated gas may rupture the case or housing and damage the equipment. In a vented system, it may result in the formation of an explosive mixture which can be ignited by arcing. It has been found that oils with high negative impulse strengths and direct current breakdown voltage values generally have very poor gassing resistance. Other electrical properties which must also be considered include the alternating current breakdown strength and the alternating current corona inception voltage. The insulating oils available heretofore have generally been deficient with respect to one or more of these properties.